The invention relates to a method for determining the age of an oxidation catalyst in an exhaust gas aftertreatment system of an internal combustion engine, a method for detecting ash in a particle filter of an exhaust gas aftertreatment system of an internal combustion engine, a control device, and an internal combustion engine.
Exhaust gas aftertreatment systems for internal combustion engines typically have a particle filter which is provided for cleaning soot particles out of the exhaust gas which is expelled by the internal combustion engine. A load of the particle filter is determined on the basis of a loading model by measuring a differential pressure dropping across the particle filter. If passive regeneration of the particle filter by nitrogen dioxide in the exhaust gas, for example during relatively long operation of the internal combustion engine in the low load range or during idling, is not sufficient, active regeneration of the particle filter is carried out, in particular as a function of the detected load, during which regeneration the soot particles are oxidized. It becomes apparent that the differential pressure level across the particle filter rises slowly in the course of the service life of the exhaust gas aftertreatment system. This rise results from two effects which are independent of one another. On the one hand, ash is increasingly deposited in the particle filter, that is to say components are deposited which cannot be burnt but instead remain permanently in the particle filter and increase the differential pressure level by blocking the particle filter; and on the other hand, aging of an oxidation catalyst which is arranged upstream of the particle filter when viewed in the direction of the exhaust gas flow causes nitrogen dioxide to be formed in the exhaust gas to a relatively low degree. This nitrogen dioxide is typically used as an oxidizing agent in the particle filter in order to be able to oxidize soot particles even in operating states of the internal combustion engine in which the exhaust gas temperature is not sufficient to bring about oxidation as a result of the residual oxygen concentration remaining in the exhaust gas. The nitrogen dioxide concentration which decreases as the aging of the oxidation catalyst progresses gives rise accordingly to reduced regeneration, with the result that the differential pressure level rises as a result of the fact that the particle filter is no longer regenerated to the same degree as in the new state of the oxidation catalyst.
These two effects, specifically the increasing depositing of ash on the particle filter, on the one hand, and the aging of the oxidation catalyst, on the other, cannot be separated from one another solely by differential pressure measurement. Therefore, in principle, it is not possible to determine on the basis of the development of the differential pressure in itself whether merely increasing depositing of ash on the particle filter is present or whether at the same time the oxidation catalyst is also exhibiting aging effects. However, it is therefore also the case that only insufficient correction of the loading model with respect to the rise in the differential pressure level is possible because it is not possible to differentiate between the contributions made by the two effects to the increase in the differential pressure. In order always to be able to reliably determine the loading of the particle filter with soot particles it is, however, necessary, on the one hand, to take into account the degree of depositing of ash on the particle filter and, on the other hand, also to take into account the regeneration of said filter in the loading model.
Conventional methods for detecting ash are carried out at operating points of the internal combustion engine at which the aging of the oxidation catalyst does not have any effect on the soot burn-off rate in the particle filter, that is to say typically at the rated power of the internal combustion engine and in the case of a high exhaust gas temperature at which the soot particles are oxidized by the residual oxygen concentration in the exhaust gas. It is clear that in this way no information whatsoever about the aging of the oxidation catalyst can be obtained because said aging does not effect the soot burn-off rate at the operating points under consideration. Adapting the loading model to the slowly rising differential pressure level therefore remains necessarily imprecise.